Since being excellent in corrosion resistance, titanium has been used for heat exchangers and condensers in plants for petrochemical industries, thermal and nuclear power generation, and seawater desalination as well as spacecraft and aircraft fields. In the above-mentioned applications, titanium is used as a welded pipe or a member obtained by welding a welded pipe and a tube plate. Particularly, those having an outer diameter of about 12 to 60 mm and a plate thickness of about 0.3 to 2.5 mm are used as a welded pipe.
In general, welding of a titanium pipe is carried out by TIG arc. TIG arc welding is advantageous in the respect that a good bead shape can be formed; however, although it depends on the plate thickness, the welding speed in the case of the above-mentioned plate thickness of 0.3 to 2.5 mm is not more than about 9 m/min (not more than 7 m/min in the case of the plate thickness of 0.5 mm, and not more than 2 m/min in the case of the plate thickness of 1.2 mm). On the other hand, laser enables deep penetration and high speed welding due to having high energy density as a heat source and laser welding can be carried out with little total heat input in a mother material and a width of a heat-affected zone by welding becomes extremely narrow. Therefore, the cooling rate of a welded metal is high and the performance of a welded part (toughness or the like) is good. However, in the case of laser welding, if key holes penetrating through a plate thickness are formed and unstably fluctuated, a current into which a gas is entrained is generated in a peripheral welding pool. If the entrained gas forms foams and the foams are solidified as they are, welding defects such as blow holes may tend to be formed in a welded part.
For example, Patent Documents 1 to 4 disclose welding methods by which weldability is improved and welding defects such as blow holes are suppressed, and which make use of the advantageous point of the laser welding that the welding speed is high. Patent Document 1 discloses a welding method employing plasma welding and laser welding. In more detail, the method improves the butt weldability (melting depth and bead width) and the welding speed by carrying out laser welding after preheating by a plasma torch. Patent Documents 2 and 3 disclose methods which prevent formation of nodular projections in the inner face of a metal pipe and occurrence of welding defects such as blow holes by carrying out laser welding after preheating by high frequency. Patent Document 4 discloses a method which prevents formation of blow holes and solidification cracking by radiating two laser beams to a molten pool at the time of laser welding.